A river in the central USA, the longest river in North America and chief tributary of the Mississippi River. Rising in the Rocky Mountains, it flows N and E through Montana, then SE across North and South Dakota before joining the Mississippi at St Louis. A series of dams provides irrigation and has considerably reduced the danger of flooding along its lower course. Length: 4367 km (2714 mi).

From its headwaters in the Rocky Mountains, the Missouri River flows 2,341 miles, meeting the Mississippi River at St. Louis, Missouri. It is America's longest river, just two and a half miles longer than the Mississippi River. The river drains about one-sixth of the United States and covers 529,350 square miles. Nearly 100 miles of undammed and unchanneled river are preserved as Missouri National Recreation River, a unit of the National Park System and part of the National Wild and Scenic Rivers System.

Missouri National Recreation River sits at the merger of glaciated and unglaciated areas of the Missouri Plateau. As with most northern high plains, the rock here was largely formed by ancient seas, including the Niobrara and Pierre Formations, which are some 80 million years old. Two plant communities line the riverbanks: mixed floodplain forest with willow and cottonwood and hardwood forests. On bluffs along the border with Nebraska, elm and oak woodland dominate. Sandbars contain early-stage growth such as grasses, sedges, and seedling willow and cottonwood. Further from the shore, forests mature, leaving cottonwood trees with an undergrowth of dogwood, sumac, wild grape, poison ivy, scouring rush, Kentucky bluegrass, and smooth brome. Hardwood forests along bluffs are communities of oak, mulberry, ash, walnut, and bur oak.

Small mammals such as mice, voles, shrews, weasels, mink, bats, rats, and ground squirrels are common along with coyote, common gray and red foxes, beavers, mule and white-tailed deer, opossums, porcupines, bobcats, gophers, mountain lions, striped skunks, cottontail and jackrabbits, river otters, woodchucks, raccoons, and badgers. Amphibians and reptiles include tiger salamanders, two species of toads, six species of frogs, snakes, and turtles. Former residents include grizzly bear, bison, and elk. Fish populations have changed due to dams that prevent migration, change habitat, and increase competition from other species. Natives include catfish, sauger, suckers, and paddlefish. Missouri National Recreation River staff manages endangered pallid sturgeon for recovery. Other sensitive species that use this habitat are interior least tern (endangered), bald eagle (threatened), and piping plover (threatened).

Before Euro-American settlement and development, the Missouri River was ecologically diverse. The natural flow of the river supported riparian areas, sandbars, islands, backwaters, sloughs, chutes, and braided channels. The river was used as a trading route by Omaha and Ponca natives, was traveled by the Lewis and Clark Expedition, and became the main waterway for trappers, traders, steamboats, and settlers. Flood control plans began in the early 20th century with two major dams, Fort Randall and Gavins Point, built in the 1940s. In its early days, the Missouri River carried a high content of sediments, earning it the nickname Big Muddy. Due to these dams, its waters now run clearer. The dams also changed the shape of the river and its shores, depleting sandbars, eroding banks, and creating a narrower channel. In addition, fewer nutrients reach fish, wildlife, and their habitat. Flood control, navigation, irrigation, hydropower, recreation, and water supply have further changed the flow and sediment in the river. Water quality and channel structure have also been affected. Within the park, engineering is not obvious, but flow and sediment regimes have been compromised. Dams and levees affect aquatic and terrestrial wildlife. Although the river will not be restored to a former state, a recovery program is underway by the U.S. Army Corps of Engineers and the U.S. Fish and Wildlife Service. The Missouri River is also affected by inflowing water from the James and the Vermillion rivers. These two rivers can contain high levels of fecal coliform bacteria that could affect recreation in the Missouri National Recreation River. Water is monitored for selenium, a natural heavy metal, ensuring that levels are within water quality regulations. Eight nonnative species of plants are under management control programs, including purple loosestrife, salt cedar, Russian olive, Canada thistle, and leafy spurge. Zebra mussels and Asian carp are nonnative fauna infesting waterways and driving out native species.

Missouri National Recreation River contains Spirit Mound, a symbolic hill associated with Native American legends contact with Euro-Americans. In 2002, a 160-acre plot was designated as Spirit Mound Historic Prairie. Restoration of this former ranchland will bring back prairie ecosystems.